1. Field of the Invention
The present invention relates to a heat exchanger utilizing a heat exchange unit into which a plurality of heat exchange plates formed of a metallic plate are combined, and especially, to an outer shell structure for such a heat exchanger, that has the minimum structural parts with which the heat exchange unit is surrounded to provide a compact structure to ensure a proper operational state of heat exchange fluids.
2. Description of the Related Art
If there is a demand that heat transfer coefficient is increased to enhance heat exchange efficiency, utilizing a heat exchanger by which transfer of heat (i.e., heat exchange) is made between a high temperature fluid and a low temperature fluid, a plate-type heat exchanger has conventionally been used widely. The plate-type heat exchanger has a structure in which a plurality of heat transfer plates are placed parallelly one upon another at prescribed intervals so as to form passages, which are separated by means of the respective heat transfer plates. A high temperature fluid and a low temperature fluid flow alternately in the above-mentioned passages to make heat exchange through the respective heat transfer plates. Japanese Patent Provisional Publication No. H3-91695 describes an example of such a conventional plate-type heat exchanger.
In the conventional plate-type heat exchanger, gasket members formed of elastic material are placed between the adjacent two plates to make the distance between them constant and define passages for fluid. However, a high pressure of the heat exchange fluid flowing between the plates may cause deformation of the gasket member, thus disabling an appropriate separation of the fluids from being ensured or leading to an unfavorable variation in distance between the plates. In such a case, an effective heat exchange may not be carried out, thus causing a problem. In view of these facts, the conventional heat exchanger involves a problem that the heat exchange fluids can be utilized only in a pressure range in which the gasket member withstands.
There has recently been proposed a heat exchanger having a structure in which metallic thin plates, which are placed at predetermined intervals, are joined together, without using any gasket members, at their ends by welding to assemble the plates into a single unit so as to form passages for heat exchange fluids, on the opposite sides of the respective plates. For example, Japanese Patent Provisional Publication No. S60-238697 describes such a heat exchanger.
The conventional heat exchanger is composed of a block body into which a plurality of plates each having edges bent at right angles are combined and welded together, and lateral walls for circulation chambers for fluids. These plates are combined together by butt-welding adjacent two plates to each other at opposite side edges thereof to prepare a combined body, and then butt-welding such a combined body and the other plate at the other opposite sides thereof, and repeating such a butt-welding operation. In addition, vertical edges projecting at each corner of the block body are directly secured to the lateral walls for forming the fluid circulation chambers, which serve as an outer shell for the heat exchanger, or secured thereto through upstanding members having rigidity, thus ensuring a state in which opening portions defined by the respective plate of the block body are separated from each other.
In the above-mentioned conventional heat exchanger, two kinds of fluids, which are to be subjected to heat exchange, flow separately from each other by means of the plates based on a cross flowing system in which flowing directions of these fluids intersect. In addition, the passage extends in a direction from one side to the other side of the plate and then turns back to the one side in the reverse direction and repeats such extension for each of a plural sets of plates as combined, thus providing an artificial flowing relationship based on a counter-flowing system or a parallel flowing system.
Japanese Patent Provisional Publication No. H3-91695 and Japanese Patent Provisional Publication No. S60-238697 describe the structure of the above-mentioned conventional heat exchanger. According to the conventional heat exchanger described in Japanese Patent Provisional Publication No. S60-238697, the plates are welded together, thus permitting to cope with heat exchange fluids having a high pressure. However, the passage extends in the direction from one side to the other side of the plate and then turns back to the one side in the reverse direction and repeats such extension, leading to a larger loss and deteriorated heat exchange efficiency in comparison with a heat exchanger having a flowing relationship in which fluids flow based on a true counter-flowing or parallel flowing system, separately from each other by means of the plates. In addition, the complicated structure of the flowing passages make such a heat exchanger unsuitable for heat exchange of gas-liquid two phase fluids, thus causing problems. Further, existence of the vertical edges for securing the sets of plates to the lateral walls requires formation of tongue portions on blank plate prior to the press formation process so as to project at corners. When a shearing process is applied to a metallic plate having a rectangular shape, useless portions thereof are usually produced, thus causing an additional problem.
Each of the conventional heat exchangers that have a structure in which plates are welded together and include an example disclosed in Japanese Patent Provisional Publication No. S60-238697 is provided on the outer side of the heat exchange unit (block) into which the plates are combined with a firm box-type shell (a pressure-resistant vessel) so as to form gaps serving as passages through which heat exchange fluids flow, between the heat exchange unit and the shell, thus preventing the fluids having high pressure from leaking out of the heat exchanger. Existence of a space for forming the passages around the unit and the box-type shell makes the whole heat exchanger considerably larger than the unit, thus causing a problem of difficulty in making the heat exchanger compact. In addition, the heat exchanger has the heat exchange unit into which there are combined the plates having a rectangular shape in which the opposite parallel sides thereof are considerably longer than the other opposite parallel sides thereof. When an inlet and an outlet of the shell for the heat exchanger, which communicate with openings provided on the longer side of the heat exchange unit, are spaced apart from each other by a long distance, the gap that is formed right outside the heat exchange unit so as to extend along the longer side thereof causes the fluid flowing from the inlet to the outlet to bypass the gaps having a high flowing resistance to directly flow into the gaps having a low flowing resistance. As a result, the fluid does not flow in the greater part of the passages through which it should flow, thus disabling a proper heat exchange relative to the other fluid from being made.